The present invention relates to an overcurrent protection element and, more particularly, to an overcurrent protection element having a gel-type encapsulant providing improved mechanical reliability and fusing characteristics.
A conventional overcurrent protection element has a fusible link suspended in a flexible resin. The flexible resin is typically a silicone resin or the like. The conductive material and dimensions of the fusible link are selected to provide a predetermined current-responsive melting characteristic where the fusible link melts at a predetermined current level. Thus, when a current flowing through the fusible link reaches the predetermined level, the fusible link melts and the current flow is prevented, thereby protecting the circuits supplied through the fusible link.
Once a fusible link is melted, the overcurrent protection device ideally remains in an open-circuit state and is replaced after a problem producing the overcurrent condition has been corrected. However, the conventional overcurrent protection element described above is subject to a condition producing a residual conductive path through the overcurrent protection element after melting of the fusible link has occurred. The conductive path is formed by the burning of the flexible resin around a melting point of the fusible link. The flexible resin is carbonized and the carbon residue creates a conductive path which bypasses the melted portion of the fusible link and thus defeats the purpose of the overcurrent protection device.
Another type of overcurrent protection device eliminates the flexible resin in order to prevent the creation of a bypassing conductive path. The flexible resin is replaced by an inorganic powder which includes glass. The glass has a sufficiently low melting point so that the glass melts when the fusible link fuses. The melted glass covers the remaining portions of the fusible link, insulating and thereby preventing the formation of a bypassing conductive path.
The inorganic powder is friable and contains air pockets which support the combustion fusion of the fusible link. Furthermore, little or no carbide is produced. However, the inorganic powder encapsulation of the fusible link exhibits undesirable mechanical properties. Due to the friability of the inorganic powder, shocks encountered during manufacture, transportation, or installation or stresses resulting from thermal expansion can cause the inorganic powder to crumble away from the fusible link. The crumbling of the inorganic powder can also apply stress to the fusible link. The fusible link thus becomes subject to fracture as a result of stresses applied and the absence of cushioning.